Method of making monocarbide fiberreinforced nickel-base superalloy composite eutectic castings

ABSTRACT

A HIGH PERFORMANCE EUTECTIC CASTING COMPRISED OF A NICKEL-BASE SUPERALLOY MATRIX REINFORCED WITH 5 TO 15 VOLUME PERCENT OF UNIFORMLY ALIGNED FIBERS OF A METAL MONOCARBIDE EXTENDING CONTINUOUSLY ENTIRELY THROUGH THE CASTING IS MADE BY DIRECTIONALLY SOLIDIFYING A MELT CONTAINING THE CONSTITUENTS OF THE MONOCARBIDE IN AMOUNTS CORRESPONDING TO THE MONOCARBIDE CONTENT OF THE EUTECTIC. THE SOLIDIFICATION RATE IS AT LEAST 1/4&#39;&#39;&#39;&#39;/HOUR.

March 26, 1974 s EI'AL 7 3,799,769

METHOD OF MAKING MONOCARBIDE FIBER-REINFORCED NICKEL-BASE SUPERALLOY COMPOSITE EUTECTIG CASTINGS Filed March 1, 1972 2 Sheets-Sheet 1 arh 26, 1974 l -.A. TARSHIS ETAL IETKOD OF MAKING MONOCARBIDE FIBER-REINFORCED NICKEL-BASE Filed flarcn 1, 1972 2 Sheets-Sheet 2 n sd a emen 3,799,769. r t METHOD OF MAKINGIMONOCARBIDE FIBER- REINFORCED NICKEL-BASE SUPERALLOY COMPOSITE EUTECTIC CASTINGS 1 Lemuel A. Tarshis, Latham, and Edward R. Buchanan, Burnt Hills, N.Y., assignors to General Electric' Co'm Filed Mar. 1, 1972, Ser. No. 230,926 Int. Cl. C22c 1/02 U.S. Cl. 75--171 5 Claims ABSTRACT OF THE DISCLOSURE A high performance eutectic casting comprised of a nickel-base superalloy matrix reinforced with 5 to 15 vol ume percent of uniformly aligned fibers of a metal monocarbide extending continuously entirely through the casting is made -by directionally solidifying amelt containing the constituents of the monocarbide in amounts corresponding to the monocarbide content of the eutectic. The solidification rate is at least A"/hou'r.

3,799,769 Patented Mar. 26, 1974 the respective phases in these eutectic products being aligned to maximize physical properties in the desired direction.

, Limitations imposed by the simple pseudobinary eutectic alloy upon the properties of these directionally-solidified composites were not avoidable until it was realized that the metal-monocarbide eutectics would withstand additions of alloying elements up to and somewhat beyond the point that the matrix qualifies as a conventional nickel-base superalloy, i.c., an alloy characterized by deliberate additions of substantial amounts of chromium to improve elevated temperature environmental resistance and optional selective additions of cobalt, aluminum, molybdenum tungsten, niobium, iron, tantalum, titanium and yttrium to provide precipitation hardening capability, improved matrix solid solution strength or both. In these complex matrix chemistry alloys, we have further found that reinforcing monocarbide fibers can be derived from tantalum, titanium, niobium, zirconium, hafnium and vanadium, and from combinations of two or more of 7 these elements with carbon. We provide carbon in these alloys in amount sufficient as a source of the metal monocarbide plus an amount in equilibrium with the monocarbide in solution in the alloy matrix.

Additionally and very importantly, we have discovered 7 that directionally-solidified, monocarbide eutectic superconcerned with a new method by which these novel castings can be produced.

In particular embodiments of this invention, these alloys have utility as directionally solidified castings forming gas turbine engine components.

CROSS REFERENCES This application is related to copending application Ser. No. 138,555, filed Apr. 29, 1971, ofIohn L. Walter and Harvey E. Cline entitled, Nickel-Base Tantalum Carbide Eutectic Alloys, assigned to the same assignee as the present invention, now abandoned; and to the continuation-in-part thereof by the same inventors, Ser. No. 153,- 006, filed June 14, 1971, entitled Nickel-Base Tantalum Carbide Eutectic Alloysjalso assigned to the same assignee as the present invention.

BACKGROUND OF THE INVENTION The physical and chemical requirements of materials of construction of gas turbine hot stage components have escalated since the advent of the aircraft jet engine because engine performance improvements require large operating temperature increases. For a long time, nickeland cobalt base alloys have served as the principal hightemperature materials of construction of jet engine buckets and vanes. The continual demand for materials of ever higher temperature capabilities has, ho'wcver,.resulted in the virtual exhaustion of alloying possibilities of the nickel-base and cobalt-base systems and has led to efforts to develop composite materials. While dispersionstrengthening o-f'nickeland cobalt-base alloys has not been fruitful, directional, solidification of pscudobinary eutectics such as Ni Al-Ni Cb has resulted in somewhat improved high-temperature mechanical strength properties (Thompson and 'Lemkey, Trans. ASM, vol. 62, page 140, 1969 Good strength properties and in addition a measure of ductility have also been obtained inthe Ni-CbC system (Lemkey and Thompson, Metal Trans, vol. 2, page 1537, June 1971) in which the typical directionallysolidified body has a relatively ductile matrix strengthened by an aligned fibrous phase. In fact, the former eutectic has been used inthe productioif'ofgas turbine buckets,

alloy castings which are free from structural inhomogeneity can be consistently produced. Thus, through critical control of the casting melt composition, one can in accordance with this invention make directionally-solidified castings in which the fibrous monocarbide structure extends continuously from one end surface to another of the cast body. The necessity for trimming the casting to eliminate non-fibrous portions can thereby be avoided without incurring any offsetting penalty of cost or product prcrformance capability. Moreover, this result can be obtained in castings of a variety of shapes and sizes with the continuous fibrous phase-forming capability being independent of casting dimensions and form. The discovery, then, is the :basis for the novel method of this invention related to that disclosed and claimed in copending applications Ser. Nos. 138,555 and 153,006 by which directionallysolidified, metal-monocanbide fiber-reinforced nickel-base superalloy composite bodies can be consistently produced. The essential difference between the present invention and that of these copending cases is that the fibrous reinforcing phase is coextensive of the casting and not limited as to length by the formation of blockyy carbide or other non-fibrous carbide phase.

SUMMARY OF THE INVENTION This invention in both its method and article of manufacture aspects is predicated upon two basic novel concepts. First, we have found that engineering limitations such as oxidation and hot-corrosion tendencies can be avoided while retaining the metal-monocarbide fiber-reinforcing eifects through additions of alloying elements to levels equivalent to and possibly beyond those of con 'ventional superalloys. Secondly, we have found that the metal-monocarbide fibrous reinforcing phase can be formed so that it is coextcnsive'with a'directionallysolidified casting by providing a casting melt containing amounts of the monocarbide constituents corresponding to the monocarbide content of the eutectic.

Thus, broadly and generally, in its method aspect, this invention comprises the steps of preparing a nickel-base superalloy casting melt containing, in addition to the superalloy matrix chemistry, the metal-monocarbide constituents in amounts corresponding to the monocarbide content of the eutectic, and directionally-solidifiying the melt at the rate of at least AW/hour. Determination of the "amounts'of the monocarbide constituents to be used in the the formulation of the casting melt in accordance with this invention can be made experimentally, as

cast body from one surface to the other of the body in its original cast condition.

DETAILED DESCRIPTION OF THE INYEINTION The melt compositions provided according to this invention are nickel-base superalloys containing at least'5.0 percent chromium and 5 to 13.5 percent of monocarbide forming metals and 0.2 to 0.6 percent carbon. These compositions, as indicated above, may also contain substantial amounts of other metals as follows:

Cobalttrace to 15.0 percent Molybdenum-trace to 5.0 percent Aluminum-trace to 7.0 percent Tungstentrace to 6.0 percent Iron-trace to 10.0 percent Titanium-trace to 1.0 percent Yttriumtrace to 1.0 percent The monocarbide-forming metal requirement of the melt compositions of this invention may be met .through the use of a single such metal or two or more of them. For example, a preferred melt composition will contain tantalum, titanium and carbon in the approximate atomic ratio 0.8:0.2:1.0, respectively, in the monocarbide. I

The volume fraction of metal monocarbide fiber in a composite eutectic casting of this invention dependsupon the kinds and amounts of optional constituents of the superalloy such as aluminum. Additionally; the "metal monocarbide content of the fibrous eutectic will beigoverned by the total composition, and particularly by the complexities of the influences of constituents of the superalloy onthercomposition of theeutetic. For these reasons, it is necessary to: determine in' some mannerrather'precisely the composition of the eutectic and the amounts of the monocarbide constituents of the eutectic so that the casting melt can be'formulated.

One procedure which has been employedsuccessfully involves preparation of a nickel-base superalloy casting melt in which the carbon and monocarbide-forming metal 1 1 contents are well into the hypereu-tectic range. Directional solidification of this melt at a suitable" rate such as Az"/hour results in an ingot orcasting containing insome increment of its length the metal monocarbide fibers. Then, chemical analyses of that segment of the casting will yield the "eutectic composition and consequently the formulation of another casting melt of this analyzed eutectic composition to be provided for directional solidificatin in accordance'with the method of this invention to produce a new article of this invention.

' The differences between the products of this invention and those resulting from the use of such hypere'utectic casting melt are apparent from the drawings accompa nying and forming a part of the specification, in which FIG. 1 comprises a photograph of an ingot directionally solidified from a hypereutectic melt, and three photomic'rographs (150 dia.) of microstructures inthreei different portions of one ingots as indicated; and,

FIG. 2 comprises aphotograph array corresponding to that of FIG. 1 in which the pictured ingot was produced in accordance with the invention by directional solidification of a casting melt containing the eutectic amounts of the monocarbide constituents. v,

The'physical properties of these new products can be further improved to an extent depending upon th'elpossibih'ty Of'ilsing the precipitatimi hardening mech.

Adam. Thus, the aluminum an d titanium contents of the superalloy have a direct bearing on the Ni Al(Ti) phase which can be developed in the matrices of these composite cast bodies. Cobalt or one example can also influence this result because it effectively increases the volume fraction of '7' which precipitates upon initial cooling or upon heat treatment ofthe casting.

One article of product of the present invent-ion is produced byv directional solidification of acasing melt consisting essentially of about: 10.0 percent chromium, 13.1 percent tantalum, 0.6 percent carbon and 76.3 percent nickel.

Another such article is produced by directional solidification of a casting melt of about: 7.9 percent chromium, 6.0 percent aluminum, 9.6 percent cobalt, 4.9 percent molybdenum, 0.4 percent titanium, 7.0 percent tantalum,

0.24 percent carbon and 63.9 percent nickel.

and distributed uniformly throughout the superalloy matrix of the composite structure of the articles in their directionally solidified condition as shown in FIG. 2. They are also free from blocky carbides and other non-fibrous forms of carbide which are characteristic of hypereutectic superalloy composite castings of this type, as illustrated in FIG. 1.

s The compositions of three typical products of this invention described in Examples I, II and III below are set out in Table I.

TABLE I.NICKEL BASE-MONOCARBIDE-REINFORCED ALLOY CASTINGS Composition of- Alldy designation Ni or" Ta 0 Al Ti 00 M0 EXAMPLE I The casting designated in, Table -I as MC-l900 was produced by directional solidification of a melt of identical composition at thesubstantially constant rate of A .hour in a temperature gradient of 250 C. per inch. Practically no. segregation was found in the solidified ingot, and the resulting .microstructure had a matrix containing throughout substantially uniformly distributed and aligned monocarbide fibers, making up approximately 5.9 volume percent of the directionally solidified ingot. The fibers were essentially single crystal monocarbides each containing essentially carbon, tantalum and titanium- The matrixv was a nickel-base superalloy containing chromium, aluminum,..molybdenum, cobalt and small amounts of tantalum, titanium and carbon essentially in equilibrium with the fibers. The'tensile properties of this alloy, as .we'll as'for the known nickel-base superalloy Ren 80,

[tested at .a strain rate of 2X10- /min., are listed in Table II for room'temperature, in Table HI at the elevated temperature ofv 1832 F., and in Table IV at the elevated temperatureof 2012 F. Y v

TABLE II.--MECHANICAL PROPERTIES AT ROOM TEM- V PERATURE Ultimate I stress Percent Alloy designation (p.s.i.) elongation v MC-o. 171,400 20.0

TABLE TIL-MECHANICAL PROPERTIES AT HIGH TEM- PERATURE 1,832 F.

Ultimate stress Percent Alloy designation (p.s.i.) elongation MC-l900 62, 800 13. 0 Ren 80 48, 000 13. 0

TABLE 1V.-MEOHANICAL PROPERTIES AT HIGH TEM- PE RATURE OF 2,0l2 F.

Ultimate stress Percent Alloy des1gnation (p.s.i.) elongation MC-l900 31, 200 14. 0 Ren 80 28,000 15. 0

TABLE V.HIGH TEMPERATURE STRESS-RUPTURE PROPERTIES Life Stress (hours to Alloy designation (p.s.i.) Temp., F. rupture) MC-1900 30, 000 1, 800 14. 3 Ren 80 30, 000 1, 800 13. 0

Table V shows the stress-rupture life of the MC-1900 alloy of the present invention at 1800 F., as compared to the known nickel-base alloy Ren 80 having relatively high stress-rupture resistance.

As may :be seen from the data of Tables II through V, the metal monocarbide-reinforced nickel-base superalloys of the present invention have comparable or superior high temperature stress-rupture resistance, with significantly superior tensile properties from room temperature to 2012 F., as compared to the known high strength nickel-base superalloy Ren 80.

EXAMPLE H The casting designated in Table I as MC-67Ti was produced by directional solidification of a melt of identical composition in the manner described in Example I. In this directionally solidified body, as in that of Example I, the monocarbides were in the form of aligned, unifomly dispersed fibers throughout the nickel-base alloy matrix, as illustrated in FIG. 2. The fibers in this instance consisted of Ta Ti C, making up about six volume percent of the directionally solidified ingot in its as-cast form.

EXAMPLE III It will be obvious to those skilled in the art upon reading the foregoing disclosure that many modifications and alterations in the specific compositions and microstructures disclosed as non-limiting examples may be made within the general context of the invention, and that numerous modifications, alterations and additions may be made thereto within the true spirit and scope of the invention as set forth in the appended claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. The method of forming a high performance eutectic casting comprising a nickel-base superalloy matrix with 5 to 15 volume percent of a fibrous phase consisting essentially of aligned fibers of tantalum-monocarbide and extending continuously entirely through the casting in its ascast condition, which comprises the steps of preparing a nickel-base superalloy casting metal melt containing constituents of the monocarbide in amounts corresponding to the monocarbide content of the eutectic, and directionally solidifying said melt at a rate of at least A1,"/hour.

2. The method of claim 1 in which the casting melt consists essentially of about 76.3 percent nickel, 10 percent chromium, 13.1 percent tantalum and 0.6 percent carbon.

3. The method of claim 1 in which the casting melt consists esentsially of approximately: 7.9 percent chromium, 9.6 percent cobalt, 4.9 percent molybdenum, 6.0 percent aluminum, 0.4 percent titanium, 7.0 percent tantalum and 0.24 percent carbon, balance essentially nickel.

4. The method of claim 1 in which the said casting melt consists essentially of about: 10.7 percent chromium, 5.9 percent aluminum, 0.6 percent titanium, 8.3 percent tantalum and 0.5 percent carbon, balance essentially nickel.

5. The method of claim 1 in which the casting melt is of composition within the following percentage ranges:

References Cited UNITED STATES PATENTS 9/ 1970 Lemkey et al. 170 7/ 1972 Tien et al. 75-171 2/1971 Thompson et al. 75171 RICHARD O. DEAN, Primary Examiner U.S. Cl. X.R. 14832, 32.5 

